Field of the Invention
The present invention relates to a hydraulic system for a vehicle having a mechanical oil pump that is driven by a prime mover such as an engine and a motor and an electric oil pump that is activated by its own electric motor.
Discussion of the Related Art
JP-A-2006-283809 describes an automatic transmission for a vehicle having an engine start/stop system and a hydraulic circuit to operate friction engagement elements such as a clutch and a brake. The hydraulic circuit has a hydraulic power source including a mechanical oil pump driven by an engine and an electric oil pump driven by its own electric motor other than the prime mover of the vehicle. A discharging side of the electric oil pump is connected to a drain passage for the mechanical oil pump through a check valve in a manner to prevent back flow of the oil discharged from the mechanical oil pump. In order to regulate pressure of the oil discharged from the electric oil pump to a predetermined level, a regulator valve is disposed between the electric oil pump and an upstream side of the check valve. When the engine is stopped by the engine start/stop system, the electric oil pump is driven to generate hydraulic pressure, and the pressurized oil is delivered to the automatic transmission.
JP-A-2009-127859 also describes a transmission having an engine start/stop system. The transmission taught by JP-A-2009-127859 also comprises a hydraulic circuit, an electric oil pump and a mechanical oil pump driven by the prime mover. According to the teachings of JP-A-2009-127859, a rotary vane pump in which oil is allowed to flow therethrough even when the pump is stopped is used as the mechanical oil pump, and a gear pump in which oil is not allowed to flow therethrough when the pump is stopped is used as the electric oil pump. JP-A-2009-127859 also describes a hydraulic circuit in which the mechanical pump is arranged parallel to the electric oil pump as a prior art.
Thus, as taught by JP-A-2006-283809 and JP-A-2009-127859, the vehicle having the engine start/stop system and the hybrid vehicle having the engine and the motor as the prime mover are provided not only with the mechanical oil pump driven by the engine but also with the electric oil pump driven by its own electric motor other than the prime mover. In those kinds of vehicles, an oil delivery can be continued by the electric oil pump even if the mechanical oil pump is stopped by stopping the engine.
In the vehicle having both mechanical oil pump and electric oil pump, a check valve and a regulator valve are used in the hydraulic system for delivering oil from those pumps to oil requiring sites. The check valve is adapted to prevent back flow of the oil discharged from one of the oil pumps. In the transmission taught by JP-A-2006-283809, the regulator valve is arranged to regulate pressure of the oil discharged from the electric oil pump to a desired level. Thus, in the vehicle having both mechanical oil pump and electric oil pump, the check valve and the regulator valve are used in the hydraulic system, and hence dimensions and cost of the hydraulic system have to be increased in comparison with conventional vehicles having no electric oil pump.
The present invention has been conceived noting the foregoing technical problems, and it is therefore an object of the present invention is to downsize a hydraulic system for a vehicle having mechanical and electric oil pumps and to reduce a cost thereof.